The Atlantic Coastal Plain is a broad, low-lying physiographic province that stretches from Cape Cod in Massachusetts to the southern tip of Florida, forming one of the most geologically distinctive and ecologically rich landscapes in North America. Beneath its flat-to-gently-rolling surface lies a wedge of sediments that thickens dramatically toward the coast, recording hundreds of millions of years of continental margin history. In 2015, the broader North American Coastal Plain was formally recognized as a global biodiversity hotspot, placing it alongside regions like Madagascar and the tropical Andes in conservation significance. That designation surprised many people, because “hotspot” is not a word most associate with the pine flatwoods and swampy lowlands of the eastern United States.
A Wedge of Sediment Thousands of Feet Deep
The Atlantic Coastal Plain owes its existence to the breakup of the supercontinent Pangaea and the subsequent opening of the Atlantic Ocean. As the continent rifted apart beginning in the Triassic and Jurassic periods, a passive continental margin formed along what is now the eastern seaboard. Rivers carried eroded material from the Appalachian Mountains and the Piedmont eastward, depositing it along this margin in layers that accumulated over tens of millions of years. The result is a seaward-dipping wedge of sedimentary rock and unconsolidated sediment that is thin at its inland edge, where it laps against the harder crystalline rocks of the Piedmont, and enormously thick near the coast.
The emerged portion of the plain sits mostly on Cretaceous and Tertiary sediments, with some deeper beds possibly dating to the Jurassic and thin Quaternary deposits blanketing the surface near the shore. At the tips of southern New Jersey and eastern North Carolina, this sedimentary wedge reaches about 10,000 feet thick. In southern Florida, it exceeds 15,000 feet.1AAPG Bulletin. Summary of Geology of Atlantic Coastal Plain The whole structure has been only mildly deformed over geologic time, giving it the characteristic “layer-cake” arrangement that geologists contrast with the tightly folded, structurally complex rocks of the neighboring Piedmont.2GeoScienceWorld Books. Late Mesozoic and early Cenozoic geology of the Atlantic Coastal Plain: North Carolina to Florida
This geology has practical consequences. The layered, porous sediments hold vast quantities of groundwater in a series of stacked aquifer systems. The Northern Atlantic Coastal Plain aquifer system alone runs from Long Island, New York, to North Carolina, and the U.S. Geological Survey has periodically refined its understanding of the hydrogeologic framework, most recently updating aquifer recharge estimates and water-use data from assessments originally completed in the 1980s.3U.S. Geological Survey. Hydrogeology and hydrologic conditions of the Northern Atlantic Coastal Plain aquifer system from Long Island, New York, to North Carolina For millions of people from the New York metro area to the Chesapeake Bay region, these aquifers are a primary source of drinking water.
Sandy Soils and What They Mean for the Landscape
If you have ever walked through a Coastal Plain forest, you probably noticed the ground underfoot: sandy, often pale, and remarkably well drained in places. The soils here are fundamentally different from those on the Piedmont just a few dozen miles inland. While the Piedmont’s crystalline bedrock weathers into clay-rich, nutrient-holding soils, the Coastal Plain’s clastic sediments produce soils that are typically two to eight meters thick with high sand content throughout. The upper layers, including both the surface horizon and the leached zone beneath it, can be one to four meters of nearly pure sand.4Geomorphology. Contrasting soils and landscapes of the Piedmont and Coastal Plain, eastern United States
These soils tend to be nutrient-poor. Studies of Coastal Plain soils have found low organic matter content and correspondingly low ability to hold onto nutrients like potassium, calcium, and magnesium.5Soil Science Society of America Journal. Potassium in Atlantic Coastal Plain Soils: I. Soil Characterization and Distribution of Potassium That might sound like a disadvantage, but these impoverished conditions are actually one of the keys to the region’s extraordinary plant diversity. Nutrient-poor soils limit the ability of any single species to dominate, creating space for a wide variety of specialized plants to coexist. Fire adds another layer to this dynamic, as we will see.
Carolina Bays and Barrier Islands
The Atlantic Coastal Plain is home to some landforms you will not find anywhere else. The most mysterious are the Carolina Bays: tens of thousands of elliptical, shallow depressions scattered across the coastal plain from New Jersey to northern Florida, with the densest concentrations in the Carolinas. Seen from the air, their shape is striking. They are oval, oriented along a roughly northwest-southeast axis, and often rimmed by low sand ridges.
Their origin has been debated for nearly a century. A recent synthesis of the evidence argues that wind was the primary force that initially scoured out the depressions in the unconsolidated sediments of the coastal plain, while wave processes within the basins shaped them into their characteristic oval form. Water-level fluctuations left behind the sand rims as ancient shorelines. The features appear to have formed and been reworked between roughly 125,000 and 11,000 years ago, coinciding with the growth and decay of the Laurentide Ice Sheet and the development of nearby sand dunes.6Marine Geology. Investigating the origin and dynamics of Carolina Bays As Holocene warming brought more rain and vegetation, the bays became quieter environments, gradually filling with fine sediment and organic matter. Most show little active geomorphic change today.
Not everyone agrees with the wind-and-wave explanation. A competing hypothesis notes that the long axes of many Carolina Bays converge near the Great Lakes region, which could be consistent with an extraterrestrial impact origin involving secondary debris from a strike on the Laurentide Ice Sheet.7Journal of Environmental & Earth Sciences. Interpreting the Geomorphology of Carolina Bays as Secondary Impact Structures The weight of geochronological evidence, however, works against an impact scenario. The bays formed over tens of thousands of years, not in a single catastrophic event, and the available dating is inconsistent with the Younger Dryas impact hypothesis or other proposed impact events.
Barrier islands are another signature feature. Along the Virginia and Carolina coasts, long, narrow sand islands run parallel to the mainland, separated from it by bays, lagoons, and salt marshes. These islands are not static. Along the Virginia coast, barrier islands have been migrating landward at rates of one to six meters per year, driven by rising sea levels. Between 1851 and 2010, this migration led to significant backbarrier area reduction and the loss of roughly 63 square kilometers of salt marsh, a decline of about 19 percent.8Geology. Barrier island migration dominates ecogeomorphic feedbacks and drives salt marsh loss along the Virginia Atlantic Coast, USA The interaction between barrier islands, salt marshes, and tidal flats creates a complex system where a change in one part ripples through the others.
A Biodiversity Hotspot Hiding in Plain Sight
When Conservation International originally mapped the world’s biodiversity hotspots in the 1990s, the Coastal Plain was not on the list. Hotspots like the Atlantic Forest of Brazil or the mountains of Southeast Asia had the kind of dramatic endemic richness that catches attention. The Coastal Plain, by contrast, felt too familiar, too close to home, too flat. But a landmark study argued convincingly that the North American Coastal Plain qualifies under the classic definition: more than 1,500 endemic plant species and more than 70 percent habitat loss.9Diversity and Distributions. How global biodiversity hotspots may go unrecognized: lessons from the North American Coastal Plain The region was subsequently recognized as the 36th global biodiversity hotspot.
Much of this plant diversity is concentrated in fire-dependent ecosystems, particularly the longleaf pine savannas that once covered enormous stretches of the southeastern Coastal Plain. Longleaf pine forests are among the most species-rich plant communities outside of tropical rainforests, with ground layers that can support 40 or more plant species per square meter in some areas. The combination of nutrient-poor soils, frequent fire, and a long growing season creates conditions that support an astonishing array of grasses, wildflowers, and legumes.
Animal diversity follows the plant diversity. A study of just five small isolated wetlands embedded within managed forest in the South Carolina Coastal Plain captured over 9,000 individual amphibians and reptiles representing 56 species across a three-year period.10Elsevier. Importance of small isolated wetlands for herpetofaunal diversity in managed, young growth forests in the Coastal Plain of South Carolina That is a remarkable concentration of herpetofaunal diversity for such a small area, and it underscores the role that even tiny wetlands play as ecological anchors in the broader landscape.
Fire as an Ecological Engine
You cannot understand the Coastal Plain’s ecology without understanding fire. Before European colonization, lightning strikes and Indigenous burning maintained vast open savannas of longleaf pine across the southeastern portion of the plain. These fires burned frequently, sometimes every one to three years, keeping hardwoods from taking over the understory and allowing sun-loving grasses and wildflowers to thrive.
The conventional wisdom in fire management has long held that a specific burning season, usually the growing season, is best for maintaining native plant diversity. But research on legume reproduction in longleaf pine savannas found a more nuanced picture. Dominant legume species showed a wide range of reproductive responses to different burn seasons, suggesting that varying the season of fire rather than sticking to a single prescribed window is more effective at maintaining a greater variety of native species.11PubMed. The effects of fire regime on legume reproduction in longleaf pine savannas: is a season selective? The takeaway for land managers is that mimicking natural variability in fire timing, rather than imposing a rigid schedule, better conserves biodiversity.
The loss of fire-dependent ecosystems has been staggering. The naval stores industry, which harvested resin from longleaf pines for tar, pitch, and turpentine, along with widespread deforestation and fire suppression, resulted in the loss of roughly 97 percent of the original longleaf woodlands and savannas.12Fire Ecology. A tree-ring record of historical fire activity in a piedmont longleaf pine (Pinus palustris Mill.) woodland in North Carolina, USA What remains today is scattered across fragments, many of them small and isolated, making restoration one of the most pressing conservation challenges on the Coastal Plain.
Salt Marshes and the Carbon They Store
The Coastal Plain’s tidal margins host extensive salt marshes, particularly along the coasts of Georgia, South Carolina, and Virginia. These marshes are ecologically productive and serve as nursery habitat for commercially important fish and shellfish. They also buffer the coastline from storm surge and filter pollutants from upland runoff.
Salt marshes are increasingly recognized for their role in carbon storage, sometimes called “blue carbon.” Globally, salt marshes store substantial amounts of organic carbon in their soils, with updated estimates putting the average at roughly 186 metric tons of organic carbon per hectare.13PubMed Central. The blue carbon of southern southwest Atlantic salt marshes and their biotic and abiotic drivers The burial rate, meaning the amount of new carbon locked away each year, averages close to 200 grams of organic carbon per square meter per year globally. These numbers are lower than some earlier estimates suggested, but they still make salt marshes among the most carbon-dense ecosystems on Earth per unit area. As sea-level rise and barrier island migration eat into Coastal Plain marshes, those carbon stores become vulnerable to release.
Thousands of Years of Human Use
People have been shaping the Atlantic Coastal Plain for millennia. The region’s rivers, estuaries, and coastline offered abundant food resources, and archaeological evidence of long-term exploitation is everywhere. Along the Rhode River sub-estuary of Chesapeake Bay, for instance, radiocarbon dating of shell middens shows that Native Americans harvested oysters and other shellfish from at least 3,200 years ago. The number of dated sites increases during the Late Woodland period, after about 1,000 years ago, likely reflecting a combination of greater site visibility, better preservation, and increased human exploitation of the watershed. Many of the shell middens accumulated rapidly, suggesting they were seasonal or logistical camps rather than permanent settlements.14Geoarchaeology. Shell Middens, Cultural Chronologies, and Coastal Settlement on the Rhode River Sub‐Estuary of Chesapeake Bay, Maryland, USA
Indigenous peoples also shaped the landscape through fire. Deliberate burning of forests and grasslands was widespread across the southeastern Coastal Plain well before European contact, maintaining the open conditions that longleaf pine and many other native species depended on. When colonists arrived and eventually suppressed fire across most of the landscape, the ecological consequences were profound, contributing to the hardwood encroachment and habitat loss that continues today.
Ghost Forests, Subsidence, and a Rising Sea
The Atlantic Coastal Plain is one of the places where climate change is most visually obvious. Along the coasts of Maryland, Virginia, and North Carolina, stands of dead trees line the edges of marshes and waterways. These “ghost forests” form when saltwater intrusion and rising water tables kill freshwater-dependent trees, replacing forest with marsh. They are among the most striking indicators of climate change along the Atlantic coast of North America.15Nature Climate Change. Sea-level driven land conversion and the formation of ghost forests
Sea-level rise on the Coastal Plain is not just a matter of water rising. The land itself is sinking. Much of the mid-Atlantic coast sits on the collapsing forebulge of the Laurentide Ice Sheet. During the last glacial period, the immense weight of the ice sheet pushed down the crust beneath it, causing areas south and east of the ice margin to bulge upward. Now that the ice is gone, that forebulge is slowly subsiding, adding to the rate of relative sea-level rise experienced along the coast.16Geophysical Research Letters. Subsidence along the Atlantic Coast of North America: Insights from GPS and late Holocene relative sea level data In places like the Chesapeake Bay region, this subsidence roughly doubles the rate of sea-level rise compared to the global average, making the area one of the most vulnerable stretches of coastline in the country.
The combined effects of rising seas and sinking land are already reshaping the Coastal Plain’s ecology. Modeling of coastal Georgia’s habitats under simultaneous sea-level rise and urban development scenarios projects severe losses for multiple habitat types by 2100. Under maximum sea-level rise and high urban growth, maritime broadleaf forest could lose 74 to 75 percent of its extent, pine forest 62 to 65 percent, and maritime shrub-scrub habitat 59 to 64 percent.17SpringerLink / PubMed Central. Effects of sea-level rise and anthropogenic development on priority bird species habitats in coastal Georgia, USA These are not distant theoretical projections; the modeling is based on growth plans already in place and sea-level scenarios from mainstream climate projections.
Restoring Longleaf on Difficult Ground
Bringing longleaf pine back to the Coastal Plain is a widely shared conservation goal, but the work is harder than planting seedlings and walking away. On hydric sites, where soils are poorly drained and standing water is common, restoration faces particular challenges. Abundant competing vegetation and poor drainage can overwhelm young seedlings. Research on hydric flatwoods sites in North Carolina has shown that site preparation, including mechanical or chemical treatment of competing plants, improved longleaf seedling growth through the first couple of years but did not significantly improve survival.18USDA Forest Service Research and Development. Site preparation for longleaf pine restoration on hydric sites: tree- and stand-level responses 15 years after planting That distinction matters for land managers: getting seedlings to grow taller is not the same as getting more of them to make it.
Restoration on drier sites tends to be more straightforward, but even there, reintroducing fire is often the harder part. Many Coastal Plain parcels are surrounded by roads, homes, and other development that makes prescribed burning complicated. Smoke management, liability concerns, and narrow weather windows constrain when and where fire can be applied. Without fire, even successfully planted longleaf forests will eventually lose the open understory and ground-layer diversity that make them ecologically distinctive.
The Fossil Record Beneath the Surface
The thick sedimentary wedge of the Coastal Plain is not just a water source and a foundation for ecosystems. It is also a rich fossil archive. The Florida Platform, the southern extension of the Coastal Plain, has produced an enormous collection of shark and ray fossils spanning from the Eocene to the Pleistocene. The Florida Museum of Natural History alone holds more than 117,000 curated specimens, representing at least 70 taxa across 10 orders and 42 genera, including 20 taxa recorded from Florida for the first time. Analysis of this collection revealed an apparent turnover along the Atlantic and Gulf Coastal Plains from a fauna dominated by lamniform sharks (the group that includes modern great whites and makos) to one dominated by carcharhiniform sharks (the group that includes modern bull sharks and hammerheads), a shift that occurred during the Eocene.19BioOne. The chondrichthyan fossil record of the Florida Platform (Eocene–Pleistocene)
Fossil hunting along Coastal Plain rivers and eroding bluffs remains popular with both professional paleontologists and amateurs. The soft, easily eroded sediments regularly expose new material after storms and high water, making sites like the Calvert Cliffs in Maryland and rivers in the Carolinas productive collecting grounds for everything from ancient whale bones to shark teeth millions of years old. The sediment layers serve as a timeline, allowing researchers to connect shifts in marine fauna to major climate events like the cooling at the Eocene-Oligocene boundary or the warming of the middle Miocene.